Conventionally, as an electromechanical transducer such as a driving element and a sensor, a piezoelectric body such as lead zirconate titanate (PZT) is used. Also, in recent years, to meet a demand for a small-sized, high-density, and low-cost apparatus, a micro electro mechanical systems (MEMS) element using a silicon (Si) substrate is increasingly used. To apply the piezoelectric body to the MEMS element, the piezoelectric body is desirably in the form of a thin film. By forming the piezoelectric body in a thin-film shape, high-precision processing using semiconductor process techniques such as film formation and photolithography is available, which enables size reduction and high density to be achieved. Also, since elements can be processed collectively on a large-area wafer, cost can be reduced. Further, electromechanical transduction efficiency is improved, which brings about advantages such as improvement in property of the driving element and improvement in sensitivity of the sensor.
As an application example of such a device using the MEMS element is applied, an inkjet printer is known. In the inkjet printer, ejection of ink is controlled while an inkjet head having a plurality of channels ejecting liquid ink is moved relatively to a recording medium such as a sheet and a cloth to cause a two-dimensional image to be formed on the recording medium.
Ejection of ink can be performed by using a pressure-type actuator (a piezoelectric type, an electrostatic type, thermal deformation, or the like) or by generating bubbles in the ink in the tube by means of heat. Among others, the piezoelectric-type actuator is advantageous in that the output force is large, the modulation is available, the response is fast, any kind of ink can be used, and the like, and is often used in recent years. In particular, to achieve a high-resolution (merely requiring a small droplet), small-sized, and low-cost printer, utilization of an inkjet head using a thin-film piezoelectric body is appropriate.
Further, in recent years, the inkjet printer is required to form a high-resolution image at higher speed. To do so, the inkjet head is required to have performance of ejecting high-viscosity ink of 10 cp (0.01 Pa·s) or higher. To achieve ejection of the high-viscosity ink, the piezoelectric thin film (ferroelectric thin film) is required to have a high piezoelectric property (piezoelectric constant d31) and a displacement generation force (film thickness of 1 μm or longer).
On the other hand, as a method for forming a piezoelectric body such as PZT on a substrate such as an Si substrate, a chemical film forming method such as chemical vapor deposition (CVD), a physical method such as sputtering and ion plating, and a liquid phase growth method such as a sol-gel method are known. The upper limit of the thickness of the thin film obtained by these methods is approximately 10 μm. When the film thickness is longer than the limit, a crack and exfoliation will be generated, and a desired property cannot be obtained.
The formed PZT exhibits an excellent piezoelectric effect when the crystal has a perovskite structure illustrated inFIG. 11. The perovskite structure is an ABO3-type crystal structure ideally having a cubic unit cell and including metal A (for example, lead) arranged at each corner of the cubic crystal, metal B (for example, zirconium or titanium) arranged at the center of the body of the cubic crystal, and oxygen O arranged at the center of each face of the cubic crystal. The perovskite-structured crystal shall include a tetragonal crystal, an orthorhombic crystal, a rhombohedral crystal, and the like into which the cubic crystal is distorted.
The PZT thin film formed on the electrode on the Si substrate is a polycrystal, which is a collective body of a plurality of crystals, due to a difference of a lattice constant from that of the crystal of the electrode. The polycrystal is formed by collecting granular crystals (granular crystals) each having a grain diameter of hundreds of nanometers or by collecting columnar crystals, each of which is a single elongated crystal grain having a width of hundreds of nanometers and extending in the film thickness direction. As for the columnar crystal, it is known that, the larger the number of crystals which have grown with the same crystal face in the film thickness direction is (the higher the degree of orientation is), the higher the piezoelectric property of the film becomes.
One of methods for improving the piezoelectric property is to add an impurity to a piezoelectric body to facilitate occurring of non-180° polarization rotation so as to improve the relative dielectric constant and the piezoelectric property. In particular, it is known that the piezoelectric body having the perovskite structure of the ABO3 type illustrated in FIG. 11 can have a high relative dielectric constant and a high piezoelectric property by adding as a donor element an element having a valence higher by one than an element located at site A or site B.
For example, in a PZT bulk ceramic, known as a donor element to be added to site A is a lanthanoid element such as lanthanum (La), bismuth (Bi), or the like, which is a trivalent cation having a valence higher by one than lead (Pb), which is a divalent ion. Also, known as a donor element to be added to site B of the PZT is niobium (Nb), tantalum (Ta), or the like, which is a pentavalent ion having a valence higher by one than each of titanium (Ti) and zirconium (Zr), which is a quadrivalent ion.
It is known that the piezoelectric property is improved when the above donor element is added to a thin-film piezoelectric body (piezoelectric thin film) instead of the bulk ceramic. For example, Non Patent Literature 1 describes that, by adding Nb to a PZT piezoelectric thin film, piezoelectric constant d31 of 250 [pm/V] or higher in terms of an absolute value is obtained.
Meanwhile, the donor additive has a higher ion valence than each of the original elements of the PZT constituting a crystal. Hence, when the additive amount increases, the ion balance is lost, and the amount of positive electric charge is likely to increase. As a result, it is known that an internal electric field caused by maldistribution of the positive electric charge is generated in the crystal.
In general, in a piezoelectric thin film with no or a small amount of donor additive, polarization and electric field hysteresis representing the relationship between the polarization amount (P) and the electric field (E) (hereinbelow referred to as P-E hysteresis) is in a symmetric shape across the vertical axis (E=0V), that is, a shape in which the polarization amounts (absolute values) on the positive and negative electric field sides are approximately symmetric, as in Patent Literature 1. However, in a piezoelectric thin film with a large amount of donor additive, it is known that the P-E hysteresis is shifted to the positive electric field side and is in a shape in which the polarization amounts (absolute values) on the positive and negative electric field sides are asymmetric. In the P-E hysteresis, an electric field when the polarization amount is zero is called a coercive electric field. In a case in which the P-E hysteresis is in a symmetric shape, the positive and negative coercive electric fields (referred to as Ec (+) and Ec (−)) are equal values. In a case in which the P-E hysteresis is shifted to the positive side, the values of the positive and negative coercive electric fields are also shifted to the positive side, and the ratio between the positive and negative coercive electric fields (Ec (+)/Ec (−)) thus increases.
Meanwhile, in an example in Patent Literature 1, a piezoelectric thin film formed on lead lanthanum titanate (PLT) serving as a seed layer seems to be La-doped PZT, but it is clearly stated that the La additive amount is zero. Accordingly, it is likely that the symmetric P-E hysteresis as illustrated in Patent Literature 1 is for PZT with no donor additive.
In a case in which a piezoelectric thin film having asymmetric P-E hysteresis is used and interposed between upper and lower electrodes to form a piezoelectric element, and in which the lower electrode is used as a common electrode while the upper electrode is used as an individual electrode, asymmetry of piezoelectric displacement due to the pole of driving voltage to be applied to the individual electrode occurs, and a high piezoelectric property can be obtained only when the driving voltage is negative (only with negative bias driving). This respect is similarly described in Patent Literature 2. That is, Patent Literature 2 describes that, when the P-E hysteresis is shifted to the positive electric field side, polarization is hard to occur even when a positive electric field is applied because the coercive electric field Ec (+) is high, the piezoelectric constant d31 (+) when a positive electric field is applied tends to be lower than the piezoelectric constant d31 (−) when a negative electric field is applied, and the piezoelectric property is thus hard to be obtained in application of the positive electric field and easy to be obtained in application of the negative electric field.
Also, for example, in Patent Literature 3, a piezoelectric actuator is driven at voltage between positive driving voltage (for example, 5V) and negative driving voltage (for example, −26V) in a range in which one of positive and negative coercive electric fields in asymmetric P-E hysteresis whose absolute value is higher is not exceeded, that is, the piezoelectric actuator is driven at lower voltage (for example, 26V in terms of an absolute value) than driving voltage required when the piezoelectric actuator is driven by a normal driving method (for example, 31V in terms of an absolute value), to attempt to reduce load to the piezoelectric body and improve driving durability and element reliability of the piezoelectric actuator.